Deformation mapping using interferometric techniques is finding increasing application in areas such as stress analysis, vibration studies, inspection of hidden flaws in structures, material property evaluation, thermal measurement and optical component testing. As used herein, the term "deformation" refers to the relative displacement of points on an object's surface relative to one another resulting from the imposed stress, as distinguished from overall, uniform displacements resulting from these stresses.
One deformation mapping technique for analyzing the overall deformation of an entire surface on an object (as opposed to relative displacement of various points on such surface) involves holographic interferometry as disclosed in U.S. Pat. No. 3,545,259 to GRANT. This technique merely measures the overall translation displacement due to deformation and rigid body motion, and provides little insight into deformations resulting from imperfections or nonuniformities in the object.
Another technique known as speckle interferometry involves reconstruction of fringe patterns wherein fringe lines represent loci of constant displacement. The so called speckle pattern is the result of the interference between light reflected from different points on the object which focus on the retina.
Still another technique sometimes referred to as shearography produces interference fringes which represent a displacement gradient, i.e., the relative displacement of two points on the surface of an object. If these two points on the object surface are moved in a uniform manner, no useful information is gained from the fringe patterns. The displacement gradient measured in shearography represents strain.
Each of the techniques described above involves a form of double exposure of a photographic plate or the like to the object before and after the latter has been stressed. However, the use of a photographic plate to record the interfering images is both time consuming and costly, particularly where the inspection process is carried out in a production environment.
As evidenced by U.S. Pat. No. 3,816,649 to BUTTERS, others have explored the possibility of using a television camera in lieu of a photographic plate to record the interfering images. The method disclosed in BUTTERS involves exposing the signal plate of the camera to an image of the object before the latter is stressed. The signal plate is then scanned by an electron beam in the conventional raster manner and the resulting video signals are digitized and stored for later use in constructing the first image. The object is then stressed and the signal plate is exposed to a second image of the stressed object. The signal plate is scanned a second time by an electron beam. The first digitized image is recalled from memory and is delivered to a differentiation circuit which compares the first and second images and produces a composite image which is displayed on a conventional CRT (cathode ray tube). Although this system essentially operates on a real time basis, providing rapid display of the interfering images, the quality of the resultant image is rather poor. The primary reason for this poor image quality involves the fact that the electron beam used in conventional raster scanning of the camera's vidicon cathode is limited to approximately 1000 lines per inch of maximum resolution. Thus, because the two images are read out at separate times and then later superimposed, the resultant image is of much poorer resolution then if the images were first superimposed on the vidicon cathode and then read out by the scanning beam. The present invention is directed toward overcoming each of the difficiencies discussed above.